What size range of droplets is ideal for applying Stokes' Law in fluid dynamics?

[smee]

Hi,
I am new to fluid dynamics and I would really appreciate some help on the subject.

When a droplet of liquid (water/blood) is moving through the air in a spherical shape, assuming the only external forces are drag and gravity, what is the range of the diameter that the drop can have so that Stokes' Law can be applied?

Thank you in advance for your help! :)

 

[arildno]

The Reynolds number is generally much too big for air that the Stokes' Law approximation is applicable.

Remember:
When viscosity goes to zero, Reynolds number goes to infinity.

 

[Chestermiller]

The Reynolds number based on the sphere diameter, relative velocity, and air properties would have to be less than 1.0

 

[arildno]

The Reynolds number based on the sphere diameter, relative velocity, and air properties would have to be less than 1.0

Which would, in effect, be only the case for the "tiniest" small spheres, because viscosity of air so small.

It shouldn't be many macroscopic droplets that obey Stokes' law in air.

 

[Chestermiller]

Which would, in effect, be only the case for the "tiniest" small spheres, because viscosity of air so small.

It shouldn't be many macroscopic droplets that obey Stokes' law in air.

Yes. We're pretty much talking about fog and cloud droplets.

 

[arildno]

Yes. We're pretty much talking about fog and cloud droplets.

Agreed. The following article from meteorology gives some typical measurements of the size distribution involved for fog droplets:
http://journals.ametsoc.org/doi/pdf/10.1175/1520-0469(1961)018<0671:AFFDSD>2.0.CO;2

while it didn't focus particularly on Stokes' law, it is probably in this range of sizes that Stokes' law is apllicable.